Vitamin A and its derivatives (retinoids) are known to exhibit potent anti-tumor activity. They are capable of retarding the appearance or reducing the incidence of tumors induced by chemical carinogenesis or virus and inhibit the growth of several transplantable tumors. These effects have been proposed to be at least partially due to stimulatory effects on the immune system. The objective of this proposal is to analyze in detail the effects one retinoid (retinoic acid) has on the immune system. We will explore the effects retinoic acid (RA) has on the in vivo and in vitro humoral response, as well as on the various cell types involved in this response. The second aim will be to study the effects this compound has on the various effector arms of the cell-mediated immune response. The response of mice to allogeneic tum r cells will be used to evaluate the effects on T killer cells and macrophages and the response to xenogeneic erythrocytes will be used to assay for antibody-dependent, cell-mediated lysis, and the delayed type hypersensitivity reaction. This study will thus provide valuable information for the proper use of RA in clinical applications of immune regulation and as a modality of treatment against neoplasia.